[Skiboot] [PATCH 3/8] cpu: add debug check in cpu_relax

[Nicholas Piggin]

Excerpts from Cédric Le Goater's message of November 26, 2021 8:39 pm:
> Hello,
> 
> On 10/3/21 03:22, Nicholas Piggin wrote:
>> If cpu_relax() is called when not at medium SMT priority, it will lose
>> the prior priority and return at medium. Add a debug check to catch
>> this, which would have flagged the previous bug.
>> 
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>>   core/cpu.c          | 6 ++++++
>>   include/processor.h | 1 +
>>   2 files changed, 7 insertions(+)
>> 
>> diff --git a/core/cpu.c b/core/cpu.c
>> index 5c10fc6e8..0f2da1524 100644
>> --- a/core/cpu.c
>> +++ b/core/cpu.c
>> @@ -80,6 +80,12 @@ unsigned long __attrconst cpu_emergency_stack_top(unsigned int pir)
>>   
>>   void __nomcount cpu_relax(void)
>>   {
>> +	if ((mfspr(SPR_PPR32) >> 18) != 0x4) {
> 
> 
> Why not use  SPR_PPR ? SPR_PPR32 is for the embedded world according
> to ISA.

Hmm I don't see that mentioned in ISA 3.1. I used PPR32 because of the
programming note in 3.1 Program Priority Registers which says "The 
ability to access the low-order half of the PPR (and thus the use of 
mfppr and mtppr) might be phased out in a future version of the 
architecture".

Thanks,
Nick